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Biogeography Questions

Bring on the tough stuff

1. What are two explanations for why the tropics have more species than the poles of the Earth?

2. What effects did the formation of the land bridge between North and South America have on marine and terrestrial species?

3. Pretend you traveled through time and met a Spanish explorer in the late 1400s, who has just discovered that the native people of the Americas have a delicious warm drink called chocolate. He brings some seeds home to Spain so that he can grow his own chocolate, but the trees just won't grow. From a biogeographical perspective, what are the explorer's options?

4. Penguins live in the Antarctic and polar bears live in the Arctic, but they use similar habitats. Why don't penguins and polar bears each live at both poles?

5. We mentioned the options species have in the face of climate change: adapt, move, or go extinct. Which of these is most likely for species on islands?

6. If you circled the globe on an east-west path, and your friend circled the globe on a north-south path (assume transportation is not a problem), who would see more similarities between all the plants in their path?

7. If the continents did not move, how could we explain disjunct distributions?

8. Which do you think are more common, endemic distribution or cosmopolitan distributions?

9. Pretend you live near a newly created volcanic island, so it is easy to pop over and check how many species are living there at any given time. New species start showing up, and the number of species goes up and up. Eventually, what will happen?

10. Many of the United States worst invasive plants come from Europe or Asia (collectively called Eurasia). Why do you think Eurasian plants are so successful in the United States?

Possible Answers

1. What are two explanations for why the tropics have more species than the poles of the Earth?

There may be more species in the tropics than in other parts of the world because of climate. The higher amounts of solar radiation support more organisms, and organisms have more opportunities to reproduce and then mutate and diverge. Another reason for higher diversity in the tropics is because the tropics have had a more consistent climate than other parts of the world—during the Ice Ages, no ice covered the tropics. Because of this, species have had more time to evolve in the tropics, rather than having to recolonize areas on the rest of the Earth.

2. What effects did the formation of the land bridge between North and South America have on marine and terrestrial species?

The land bridge allowed terrestrial species to expand their ranges onto a new continent. Northern species moved south and southern species moved north, but most of the southern species died out while the northern species survived. Marine organisms could no longer interact with organisms of their same species on both sides of the land bridge. Pacific organisms started to differentiate themselves from Atlantic organisms. Meanwhile, the Atlantic Ocean got warmer and saltier.

3. Pretend you traveled through time and met a Spanish explorer in the late 1400s, who has just discovered that the native people of the Americas have a delicious warm drink called chocolate. He brings some seeds home to Spain so that he can grow his own chocolate, but the trees just won't grow. From a biogeographical perspective, what are the explorer's options?

Cacao is a tropical understory tree, and Spain has a Mediterranean climate. The cacao trees probably need more moisture and shade than Spain has to offer. The explorer could keep exporting chocolate from America, or take some to grow in a tropical location closer to home. Historically, both things happened—chocolate was harvested from the New World and brought to Europe, and Europeans established plantations in Africa.

4. Penguins live in the Antarctic and polar bears live in the Arctic, but they use similar habitats. Why don't penguins and polar bears each live at both poles?

It is mostly a matter of history. The ancestor of polar bears lived much farther north than the ancestors of penguins. Polar bears and penguins are both adapted to cold, icy climates, but probably were not able to disperse the long distances between the north and south poles because they would have had to go through unsuitable habitat (temperate and tropical zones). If you look at the map of the continents through time, you can see that the landmasses that are now in the Arctic were never close to the landmasses that are now in the Antarctic. There was no opportunity for these animals to move from one to the other.

5. We mentioned the options species have in the face of climate change: adapt, move, or go extinct. Which of these is most likely for species on islands?

Islands are limited in the input of new organisms (the immigration rate), especially if they are far from the mainland. On isolated islands, many species will probably go extinct. However, if there is enough genetic diversity among the individuals there, island species may persist through adaptations. The problem for them is they really have nowhere to move, so they are left with two options: adapt or die.

6. If you circled the globe on an east-west path, and your friend circled the globe on a north-south path (assume transportation is not a problem), who would see more similarities between all the plants in their path?

You would see more similar plants than your friend. You would stay on the same latitude the whole trip. Your friend would pass many latitudes. The big differences in climate are latitudinal, meaning they change north to south, not east to west. Though there would be differences in climate as you passed through mountains and coastal areas, overall the plants would be much more similar along an east-west path than a north-south path.

7. If the continents did not move, how could we explain disjunct distributions?

If the continents did not move, we probably would not have as many disjunct distributions. But we would have to use the same logic our ancestors did—if two closely related organisms live in completely separate places, they either dispersed from one place to the other, or they must have been connected at one point by a land bridge that disappeared with sea level changes. The other thing to check would be if the species or genera in question are actually closely related—if they just look alike, it could be a case of convergent evolution.

8. Which do you think are more common, endemic distribution or cosmopolitan distributions?

The answer may surprise you: there are more endemic species than cosmopolitan species. Even though many endemic species are adapted to certain conditions, they just live in one place, so they only have to deal with conditions in one place. Cosmopolitan species need to have a lot of tricks up their sleeves, because they have to deal with a huge range of environmental conditions and competitors from all over the globe. As they say, it's lonely at the top. The most common type of distribution is somewhere in between endemic and cosmopolitan, occurring in multiple places but not everywhere.

9. Pretend you live near a newly created volcanic island, so it is easy to pop over and check how many species are living there at any given time. New species start showing up, and the number of species goes up and up. Eventually, what will happen?

Eventually or even right away, the species living on the island will compete for resources. Some species will go extinct from the island and others will show up, but at some point the number of species will stay the same, even though new ones are showing up and other ones are dying out. Despite the turnover, there will be an equilibrium number based on how far the island is from the mainland and how big the island is.

10. Many of the United States worst invasive plants come from Europe or Asia (collectively called Eurasia). Why do you think Eurasian plants are so successful in the United States?

The United States shares a few things with Europe and Asia: northern latitudes and a shared history. The northern latitudes give each continent its temperate climate. There are tropical, sub-tropical and arctic climates in all these places, but the vast majority is temperate. That means that a species coming from Europe is likely to find an agreeable climate in the US. These three continents were also part of Laurasia together, meaning there may be similarities between what lives in each continent anyway.